Thermal interactions between solid walls and oscillating gas produce a rich variety of thermoacoustic phenomena such as self-sustained gas oscillations and acoustic heat pumping. The energy conversion between heat flow and work flow occurs in a stack of plates, resulting in the two classes of engines. One is the prime mover (thermoacoustic engine) and the other is the heat pump. These engines are divided into two groups called "standing wave thermoacoustic engine" and "traveling wave thermoacoustic engine". This work is to experimentally study basic properties of such thermoacoustic engines.A spontaneous gas oscillation in a resonator equipped with a differentially heated stack of plates is studied experimentally by measuring the pressure and velocity of the gas. Work done by gas parcels is observed to flow out of stack with a temperature gradient. This paper reports the first measurements of the work flow and the work source in the standing wave thermoacoustic oscillation. Results are compared with the computer simulation based on the thermoacoustic theory and provide strong support for the basic ideas of standing wave thermoacoustic engines.We have built a thermoacoustic engine consisting of a differentially heated stack of plates in a looped tube and observed spontaneous gas oscillations of the traveling wave mode running around the loop. Stability boundary and thermally produced acoustic power are compared with those for the engine tested in a resonator. The engine in a looped tube acts as a traveling wave power amplifier, whose onset temperature rations are significantly smaller than those for the engine in a resonator.